Great impetus was given to the biological investigation of disease by
the discoveries which led to the formulation of the cell theory in
1840 and the brilliant work of Pasteur on fermentation,[1] but it was
not until 1878 that it was definitely proved that a disease of cattle
called anthrax was due to a species of bacteria. What should be
regarded as such proof had been formulated by Henle in 1840. To prove
that a certain sort of organism when found associated with a disease
is the cause of the disease, three things are necessary:

1. The organism must always be found in the diseased animal and
associated with the changes produced by the disease.

2. The organism so found must be grown outside of the body in what is
termed pure cultures, that is, not associated with any other
organisms, and for so long a time with constant transfers or new
seedings that there can be no admixture of other products of the
disease in the material in which it is grown.

3. The disease must be produced by inoculating a susceptible animal
with a small portion of such a culture, and the organism shown in
relation to the lesions so produced.

It is worth while to devote some attention to the disease anthrax.
This occupies a unique position, in that it was the first of the
infectious diseases to be scientifically investigated. In this
investigation one fact after another was discovered and confirmed;
some of these facts seemed to give clearer conceptions of the disease,
others served to make it more obscure; new questions arose with each
extension of knowledge; in the course of the work new methods of
investigation were discovered; the sides of the arch were slowly and